Given the proportion of adolescents affected by unipolar depression, accurate assessments are necessary to identify those likely to be in need of care. While there is reason to believe that depression manifests itself differently throughout development, we know of no instruments that are tailored expressly to adolescents. Certain depressive symptoms may be more apparent or salient for adolescents of different ages, or in various subgroups based on gender, substance use and physical health. Item response theory provides a useful framework for the development of a flexible measurement system that is sensitive to these influences. Using existing data from 3 large nationally representative adolescent samples, the proposed developmental study will adopt an item response theory approach to adolescent measurement development and refinement. Project aims are to: 1. Establish overlapping, age-specific item sets for each scale (using IRT analyses) that incorporate information about potential vulnerabilities to depression (i.e., gender, physical health and substance use status) by: a) identifying scale items that can be considered common across age groups and scale items that should be age-specific; and b) identifying scale items that should be treated differently for people in one or more of these vulnerable groups; and 2. Link data sets and compare items in order to develop a prototype set of age-specific assessments: a) compare the potential utility of depression items from each of the scales in the development of overlapping age-specific assessments (using IRT analyses) by linking the data sets and generating item characteristics for all items simultaneously; and b) use this information to develop a prototype set of age-specific assessments that appropriately cover the vegetative, cognitive, and affective aspects of depression; are sensitive to possible vulnerabilities associated with gender, physical health, and substance use status; and are linked through common items so that scores can be compared across assessments. Successful completion of these aims will result in a prototype set of age-specific assessments for adolescents that uniquely combines the most sensitive and relevant items from the existing studies. Subsequent fielding of this instrument set (to be proposed as a follow-on R01) will allow for evaluation and refinement of the prototype items, establishment of diagnostic norms, and development of a portable scoring system.